def f(a):
    for i in range (2,a):
        if a%i==0:
            return 0
    return 1
a=int(input())
if a<1000 or a>=10000:
    print(0)
else:
    if f(a):
        M=a
    else:
        M=0
        while a!=0:
            x=a%10
            y=a%100
            z=a%1000
            if f(x): M = max(M,x)
            if f(y): M = max(M,y)
            if f(z): M = max (M,z)
            a//=10
    print(M)
    
